1. Field of the Invention
The present invention relates to an ultrasonic wave medical treatment apparatus for treating treatment targets such as tumors, calculi, etc. inside a living body by applying intense ultrasonic waves from an outside of the living body or a body cavity of the living body, under the guidance of the magnetic resonance imaging (MRI).
2. Description of the Background Art
In recent years, for a treatment of the calculosis, much attention has been attracted to a lithotriptor for destroying calculi inside a living body non-invasively by externally applying intense ultrasonic waves focused on the calculi.
Also, for a treatment of the tumors, much attention has been attracted to a hyperthermia for heating the tumor tissues at the temperature over 42.5.degree. C., and a thermal treatment for causing a thermal degeneration by heating at the high temperature over 60.degree. C.
In order to carry out these treatments, there are many propositions for an apparatus for focusing the intense ultrasonic waves generated outside of the living body onto a treatment target portion within the living body, and thermally treating a cancer by the heat generation of the cancer tissue due to the absorption of the ultrasonic energy, such as those disclosed in U.S. Pat. No. 4,620,546, and Japanese Patent Application Laid Open No. 5-137733 (1993). This latter reference proposes a unified configuration of the lithotritor and a thermal treatment apparatus, noting that these tow apparatuses have similar structures.
On the other hand, the researches are also carried out for a treatment method to kill the cancer tissue by the mechanical force of the pulse shaped shock waves having sufficient intensity to destroy the calculi which are irradiated onto the cancer, as disclosed in Hoshi, S. et al.: "High Energy Underwater Shock Wave Treatment on Implanted Urinary Bladder Cancer in Rabbits", Journal of Urology, Vol. 146, pp. 439-443, August, 1991.
Now, in positioning the focus in such a cancer treatment apparatus, the two dimensional ultrasound tomographic images are usually utilized, but this use of the two dimensional tomographic images makes it very difficult to carry out a thorough treatment of the entire tumor as the actual tumor often has a complicated three dimensional shape. For this reason, there has been a proposition to employ the three dimensional ultrasound images instead of the ultrasound tomographic images as disclosed in European Patent No. 0 194 897.
However, in the ultrasound images, the region behind the pneumatic organs such as the bones and the lung becomes invisible, so that the accurate three-dimensional information cannot be obtained even when the three dimensional ultrasound images are utilized.
Furthermore, in the conventional ultrasonic wave medical treatment apparatus, only the relative position of the focal point and the treatment target portion can be ascertained at best, and there has been no means for judging the effect of the treatment, so that the decision for the continuation or termination of the treatment cannot be made until several weeks to several months after the treatment. For these reasons, there has been a proposition for an ultrasonic wave medical treatment apparatus incorporating the MRI or the X-ray CT (computed tomography) as disclosed in Japanese Patent Application Laid Open No. 5-300910 (1993).
In this regard, it is known that the tissue degeneration due to the heat can be confirmed by taking the T2 weighted images using the MRI, as reported in Jolesz, F. A. et al.: "Laser Surgery Benefits from Guidance by MR", Diagnostic Imaging, pp. 103-108, September 1990. Consequently, by observing the difference between the T2 weighted images taken before and after the treatment, it becomes possible to judge the effect of the treatment, so that the treatment can be carried out while checking the untreated portion and the sufficient treatment effect can be secured by a minimum amount of the shock wave irradiation.
It is also possible to set up a treatment plan concerning the scanning method and range for the shock wave focal point, and the intensity, period, and interval for the shock wave irradiation, according to the frozen image obtained by the MRI. Here, however, even when such a treatment plan is prepared, the accurate treatment cannot be expected unless the accurate positioning of the shock wave focal point is guaranteed.
In the conventional ultrasonic wave medical treatment apparatus, it has been necessary to remove the ultrasonic wave applicator from the patient at a time of moving the patient in and out of the MRI gantry, due to the mechanism for moving the ultrasonic wave applicator and the structural properties of the treatment bed and the MRI gantry.
For example, at the beginning of the treatment, after the MR images is taken before the treatment in order to set up the treatment plan, the patient is moved out of the MRI gantry once in order to attach the ultrasonic wave applicator, and then after the positioning of the intense ultrasonic wave focal point with respect to the treatment target portion is made by using the MR images and the ultrasound images, the actual treatment is started. In addition, in a case of carrying out the treatment while judging the treatment effect and checking the untreated portion by the MR images, it is necessary to repeat the operation in which the ultrasonic wave applicator is removed from the patient once and the patient is moved into the MRI gantry in order to take the MR images, and after the treatment effect is judged, the patient is moved out of the MRI gantry again in order to attach the ultrasonic wave applicator, and then after the positioning of the ultrasonic wave focal point is re-established, the treatment is resumed.
In re-establishing the positioning of the ultrasonic wave focal point, even when the relative position of the ultrasonic transducer and the treatment target portion is memorized accurately at a time of the initial positioning, the focal point position can be displaced by a slight movement of the patient. In particular, when it is necessary to repeat the attaching and removing of the ultrasonic wave applicator for a number of times, the probability for the focal point position to be displaced from a desired position becomes large.
Moreover, when the ultrasonic wave applicator is simply pressed against the body surface of the patient, there is a danger for the body surface to move with respect to the ultrasonic wave applicator due to the respiration movement.
Now, there is a recent proposition for mounting the ultrasonic transducer on a catheter, and inserting this catheter into the patient's body under the guidance of the MRI to establish the positioning of the ultrasonic transducer and the treatment target portion, so as to treat the treatment target portion by irradiating intense ultrasonic waves from the ultrasonic transducer mounted on the catheter, as disclosed in Japanese Patent Application Laid Open No. 4-53533 (1993).
In this proposition, when the receiving system of the MRI is for the entire body, the S/N ratio becomes insufficient for the treatment plan set up, the accurate treatment effect judgement, and the real time treatment monitoring, so that it is necessary to use a surface coil to be placed on the body surface in order to obtain the MR images at a sufficiently high S/N ratio. However, because of the presence of the ultrasonic wave applicator on the body surface near the treatment target portion, it is impossible to place this surface coil on the body surface near the treatment target portion during the ultrasonic wave medical treatment. Also, when a surface coil is used for the receiving system of the MRI, the positioning of the receiving system to image the desired treatment target portion at a high S/N ratio becomes difficult as the surface coil has a relatively large sensitivity fluctuation. Moreover, when the ultrasonic transducer is mounted on the catheter and the ultrasonic waves are irradiated from a body cavity, the surface coil cannot be position near the treatment target portion, so that the sufficient MR images of the treatment target portion cannot be obtained.
On the other hand, in the conventional piezoelectric type ultrasonic wave medical treatment apparatus, the focal point is extremely small, so that in the treatment method such as that for causing the thermal degeneration on the tissues by heating the localized region at a high temperature over 80.degree. C. or that for destroying the tissues mechanically by the shock waves, the displacement of the focal point position can lead to the destroying of the normal tissues, unlike the treatment method such as the hyperthermia which carries out the treatment by utilizing the difference in the thermal sensitivity of the tissues. For this reason, it has been necessary to make a highly accurate positioning in the conventional piezoelectric type ultrasonic wave medical treatment apparatus, but there has been a danger that the treatment target portion can be moved due to the respiration or the body movement of the patient, or that the focal point position can be shifted due to the reflection of the ultrasonic waves at the body surface.
In addition, as the focal point is de-focused by the reflection of the ultrasonic waves, there has been a possibility that the temperature at the focal point does not reach to an expected level or that the treatment becomes insufficient as the intensity of the shock waves becomes insufficient. As a consequence, the burden on the patient as well as the physician can be increased by the re-treatment required by the recurrence of the cancer due to the insufficient treatment. Furthermore, there is a danger than the treatment in an accurate range cannot be made as the focal point size becomes larger due to the de-focusing of the focal point.
There is also a need to take an impedance matching between the driving circuit and the ultrasonic transducer in the ultrasonic wave medical treatment apparatus. However, because the piezoelectric element used as the ultrasonic transducer has a high Q at the mechanical resonance point, the impedance matching between the piezoelectric element and the amplifier can be displaced during the treatment due to the change of the characteristic caused by the heat generation of the piezoelectric element, such that there is a danger for failing to obtain the expected acoustic output.
Also, due to the displacement of the impedance matching, the reflected electric power of the ultrasonic transducer can be increased such that there is a possibility for the electro-acoustic conversion efficiency to be deteriorated.
Moreover, in a treatment method in which the malignant tumor tissue located at the focal point is killed by heat, the negative pressure at the focal point becomes large as the focal point input power is large, such that the stable cavitations can be generated and grown as the intense ultrasonic waves are applied continuously, and there is a possibility that the sufficient power cannot reach to the intended focal point due to the scattering of the ultrasonic waves by the cavitations. In addition, there has been a possibility for the appearance of a hot spot at an unexpected location due to the heat generation by the scattered ultrasonic waves.